Abstract

Stormont, Fletcher's Adit and Ti Tree Creek are three mineralised, calc-silicate skarnsin the Moina district, NW Tasmania. Stormont is a Au and Bi-bearing skarn, Fletcher's Aditcontains minor Cu, Au, Bi, W, Sn and Zn, and Ti Tree Creek contains small amounts of Snand Bi. The three skarns are hosted in the basal section of the Ordovician Gordon Limestoneand the upper portion of the underlying Late Cambrian to Early Ordovician Moina Sandstone.Metasomatism and mineralisation occurred during the late Devonian intrusion of the ilmeniteseries, Dolcoath Granite. The granite-to-prospect distance increases to the west, from Ti TreeCreek to Fletcher's Adit, to the most distal (0.5 - 1 km) Stormont deposit.The three skarn deposits have a similar paragenesis during the metamorphic,metasomatic and early retrograde stages of skarn development Wollastonite and saliticclinopyroxene were formed during the metamorphic stage of skarn development. Andradite(and minor Cr-rich grossular) garnet, salitic clinopyroxene and minor vesuvianite were formedduring the infiltration metasomatic phase.Epidote, actinolitic amphibole, quartz and fluorite were the major phases formedduring the actinolite replacement stage of retrograde alteration, while chlorite, fluorite,muscovite, calcite and quartz were stable during the later mineralisation stage of retrogradealteration. Differences between the deposits in the late retrograde and mineralisation stagesinclude the intensity of retrograde alteration, the abundance of minor retrograde phases, nativegold, native bismuth, bismuthinite, galenobismutite, magnetite and base metal sulphides.Opaque minerals present in the three skarns are magnetite, pyrrhotite, pyrite,marcasite, arsenopyrite, native bismuth, bismuthinite, galenobismutite, a Bi-Te sulphide,chalcopyrite, native gold, hematite and goethite. Gold is associated with bismuthinite.Generally, Au, Bi, Pb and Sn mineralisation is restricted to the calcareous host rocks, Cu, W,As, Ag and Zn mineralisation occurs in the skarn or the footwall lithologies and Momineralisation occurs exclusively in the footwall.Stormont is a gold skarn, having a high pyroxene/garnet ratio, a deficiency of basemetal sulphides, abundant retrograde alteration, late stage reduced mineralising fluids,statistically significant positive Au/Bi, Au/Pb and Bi/Pb metal correlation coefficients, a statistically insignificant positive Au/Cu metal correlation coefficient, locally high Au and Bigrades, very high Au(ppm)/Cu(%) ratios, and common bismuthinite and native bismuth.The compositions of major calc-silicate phases support Stormont's status as a goldskarn. Clinopyroxenes from Stormont belong to the diopside-hedenbergite series, rangingfrom Diso to Di76· although they are occasionally anomalously manganiferous (up to J09), andare not highly aluminous. Garnets from Stormont are grandites and range from A£49 to Ad6Q.The observed opaque mineral paragenesis and thermodynamic modelling indicate thatgold was deposited (with bismuthinite) as a bisulphide complex late in the paragenesis, as aresult of the mineralising fluids dropping in temperature and f02. A discrete population of highAu and Bi grades is unique to Stormont in the Moina area, indicating that the mineralisingfluids that precipitated Au and Bi at Stormont were of a different character to those atFletcher's Adit.The gold at Stormont is hypothesized to have originated from underlying Cambrianvolcanics which were leached by late stage, circulating meteoric (and magmatic?) fluids.Stormont's increased granite-to-prospect distance, fracture/fault controlled permeability andinter-granular permeability of the skarn (due to massive actinolite replacement), as well as theregionally high permeability of the footwall arenites may have assisted the development of anefficient large-scale convection cell.

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